1. Field of the Invention
This invention relates to neutron detectors, methods of making same, and in particular, to high-efficiency neutron detectors and methods of making same.
2. Background Art
Semiconductor detectors coated with neutron reactive materials offer an alternative approach to scintillator-based neutron imaging devices for neutron radiography (normally scintillating screens coupled to photographic film or to other photorecording devices). The detectors also offer an alternative to He-3 proportional counters, BF3 proportional counters, Li-loaded glasses, and other scintillator-based systems for neutron detection. Neutron reactive film-coated devices investigated in previous works include Si, SiC, GaAs, and diamond detectors, all of which have advantages and disadvantages as described in references 1–6 noted at the end of this section.
The converter films attached to semiconductor devices most often used for neutron-detection utilize either the 6Li(n,α)3H reaction or the 10B(n,α)7Li reaction. Due to low chemical reactivity, the most common materials used are pure 10B and 6LiF. Neutron reactive films based on the 157Gd(n,γ)158Gd reaction show a higher neutron absorption efficiency than 10B(n,α)7Li and 6Li(n,α)3H-based films, however the combined emission of low energy gamma rays and conversion electrons from 157Gd(n,γ)158Gd reactions make neutron-induced events difficult to discriminate from background gamma-ray events. As a result, Gd-based films are less attractive for devices where background gamma ray contamination is a problem. Alternatively, the particle energies emitted from the 6Li(n,α)3H and the 10B(n,α)7Li reactions are relatively large and produce signals easily discernable from background gamma ray noise. Thus far, thermal neutron-detection efficiencies have been limited to only 4% for 6LiF and 10B single-coated devices.
Expected Efficiency of Conventional 10B and 6Li Coated Detectors
The 10B(n,α)7Li reaction leads to the following reaction products, as described in reference 7 noted at the end of this section:
                             10            ⁢      B        +                           0        1            ⁢      n        ->          ⁢                                                                                Reaction            ⁢                                                  ⁢            Q            ⁢                          -                        ⁢            Value                    _                                              {                                                                                                                                                                                             7                                                ⁢                        Li                                            ⁡                                              (                                                  at                          ⁢                                                                                                          ⁢                          1.015                          ⁢                                                                                                          ⁢                          MeV                                                )                                                              +                                          α                      ⁡                                              (                                                  at                          ⁢                                                                                                          ⁢                          1.777                          ⁢                                                                                                          ⁢                          MeV                                                )                                                                              ,                                                                                                                                                                        Li                                                *                                                                            7                                            ⁡                                              (                                                  at                          ⁢                                                                                                          ⁢                          0.840                          ⁢                                                                                                          ⁢                          MeV                                                )                                                              +                                          α                      ⁡                                              (                                                  at                          ⁢                                                                                                          ⁢                          1.470                          ⁢                                                                                                          ⁢                          MeV                                                )                                                                              ,                                                                                                                                                      2.792                  ⁢                                                                          ⁢                                      MeV                    ⁡                                          (                                              to                        ⁢                                                                                                  ⁢                        ground                        ⁢                                                                                                  ⁢                        state                                            )                                                                                                                                                                                                2.310                ⁢                                                                  ⁢                                  MeV                  ⁡                                      (                                          1                      ⁢                      st                      ⁢                                                                                          ⁢                      excited                      ⁢                                                                                          ⁢                      state                                        )                                                                                          which are released in opposite directions when thermal neutrons (0.0259 eV) are absorbed by 10B. After absorption, 94% of the reactions leave the 7Li ion in its first excited state, which rapidly de-excites to the ground state (˜10−13 seconds) by releasing a 480 keV gamma ray. The remaining 6% of the reactions result in the 7Li ion dropping directly to its ground state. The microscopic thermal neutron absorption cross section is 3840 barns. Additionally, the microscopic thermal neutron absorption cross section decreases with increasing neutron energy, with a dependence proportional to the inverse of the neutron velocity (1/v ) over much of the energy range, as described in references 8 and 9.
The 6Li(n,α)3H reaction leads to the following products:
                                                          Reaction          ⁢                                          ⁢          Q          ⁢                      -                    ⁢          Value                _                                                                                                                                        6                                ⁢                Li                                      +                                                           0                1                            ⁢              n                                ⁢                      ->            3                    ⁢                                    H              ⁡                              (                                  at                  ⁢                                                                          ⁢                  2.73                  ⁢                                                                          ⁢                  MeV                                )                                      +                          α              ⁡                              (                                  at                  ⁢                                                                          ⁢                  2.05                  ⁢                                                                          ⁢                  MeV                                )                                                    ,                                      4.78          ⁢                                          ⁢          MeV                                              which again are oppositely directed if the neutron energy is sufficiently small. The microscopic thermal neutron (0.0259 eV) absorption cross section is 940 barns. The thermal neutron absorption cross section also demonstrates a 1/v dependence, except at a salient resonance above 100 keV, in which the absorption cross section surpasses that of 10B for energies between approximately 150 keV to 300 keV, as described in references 8 and 9. Additional resonances characteristic to either isotope cause the absorption cross section to surpass one or the other as the neutron energy increases. Due to its higher absorption cross section, the 10B(n,α)7Li reaction leads to a generally higher reaction probability than the 6Li(n,α)3H reaction for neutron energies below 100 keV. However, the higher energy reaction products emitted from the 6Li(n,α)3H reaction lead to greater ease of detection than the particles emitted from the 10B(n,α)7Li reaction.
The term “effective range” (denoted L) is the distance through which a particle may travel within the neutron reactive film before its energy decreases below the set minimum detectable threshold, or rather, before its energy decreases below the electronic lower level discriminator (LLD) setting. The term does not take into account additional energy losses from contact “dead regions.” The neutron reaction products released do not have equal masses, and therefore do not have equal energies or effective ranges. Neutrons may interact anywhere within the reactive film, and the reaction products lose energy as they move through the neutron reactive film. Reaction product self-absorption reduces the energy transferred to the semiconductor detector, and ultimately limits the maximum film thickness that can be deposited over the semiconductor device. The measured voltage signal is directly proportional to the number of electron-hole pairs excited within the semiconductor. Reaction products that deposit most or all of their energy in the detector will produce much larger voltage signals than those reaction products that lose most of their energy before reaching the detector.
The energy absorbed in the detector is simply the original particle energy minus the combined energy lost in the boron film and the detector contact during transit. At any reaction location within the reactive film, a reduced energy will be retained by either particle that should enter the detector, being the maximum possible if the trajectory is orthogonal to the device contact. Hence, if the interaction occurs in the 10B film at a distance of 0.5 μm away from the detector, the maximum energy retained by the 7Li ion when it enters the detector will be 430 keV, and the maximum energy retained by the alpha particle will be 1250 keV, as described in references 10 and 11. For the same interaction distance of 0.5 μm from the detector, the energy retained by the particle when it reaches the detector decreases as the angle increases from orthogonal (>0°). Given a predetermined minimum detection threshold (or LLD setting), the effective range (L) for either particle can be determined For instance, an LLD setting of 300 keV yields LLi as 0.810 microns and Lα as 2.648 microns, as described in references 10 and 11. Similar conditions exist for 6LiF and 6Li films.
A commonly used geometry involves the use of a planar semiconductor detector, generally indicated at 10, over which a neutron reactive film 11 has been deposited, as shown in FIG. 1. Upon a surface of the semiconductor detector is attached a coating that releases ionizing radiation reaction products 12 upon the interaction with a neutron 13. The ionizing radiation reaction products 12 can then enter into the semiconductor material 14 of the detector 10, thereby creating a charge cloud 15 of electrons and “holes,” which can be sensed to indicate the occurrence of a neutron interaction within the neutron sensitive film. The charges 15 are swept through the detector through the application of a voltage, which is applied through the use of conductive contacts 16 and 17 upon the surfaces of the semiconductor detector, where the surfaces are generally parallel to each other.
Assuming that the neutron beam is perpendicular to the detector front contact 16, the sensitivity contribution for a reaction product species can be found by integrating the product of the neutron interaction probability and the fractional solid angle, defined by the reaction product effective ranges subtending the device interface, as described in references 10 and 11, which yields:
                                                        S              p                        ⁡                          (                              D                F                            )                                =                      0.5            ⁢                          F              p                        ⁢                          {                                                                    (                                          1                      +                                              1                                                                              ∑                            F                                                    ⁢                          L                                                                                      )                                    ⁢                                      (                                          1                      -                                              ⅇ                                                  -                                                                                    ∑                              F                                                        ⁢                                                          D                              F                                                                                                                                            )                                                  -                                                      D                    F                                    L                                            }                                      ⁢                                  ⁢                                            for              ⁢                                                          ⁢                              D                F                                      ≤            L                    ,          and                                    (                  1          ⁢          a                )                                                                    S              p                        ⁡                          (                              D                F                            )                                =                      0.5            ⁢                          F              p                        ⁢                          ⅇ                              -                                                      ∑                    F                                    ⁢                                      (                                                                  D                        F                                            -                      L                                        )                                                                        ⁢                          {                                                                    (                                          1                      +                                              1                                                                              ∑                            F                                                    ⁢                          L                                                                                      )                                    ⁢                                      (                                          1                      -                                              ⅇ                                                  -                                                                                    ∑                              F                                                        ⁢                            L                                                                                                                )                                                  -                1                            }                                      ⁢                                  ⁢                                            for              ⁢                                                          ⁢                              D                F                                      >            L                    ,                                    (                  1          ⁢          b                )            where ΣF is the macroscopic neutron absorption cross section, DF is the film thickness, and FP is the branching ratio of the reaction product emissions. The total sensitivity accordingly can be found by adding all of the reaction product sensitivities:
                                                        S              ⁡                              (                                  D                  F                                )                                      ⁢                          ❘              Total                                =                                    ∑                              p                =                1                            N                        ⁢                                          S                p                            ⁡                              (                                  D                  F                                )                                                    ,                            (        2        )            where N is the number of different reaction product emissions. In the case of 10B-based films, N equals 4. Notice from equation 1b that the value of SP reduces as DF becomes larger than the value of L. As a result of this, there will be an optimum neutron reactive film thickness for front-irradiated detectors. Since the minimum particle detection threshold determines the effective range (L), the optimum film thickness is also a function of the LLD setting. For example, with the LLD set at 300 keV, the maximum achievable thermal neutron-detection efficiency is 3.95%. The thermal neutron-detection efficiency can be increased to 4.8% by lowering the LLD setting, but only at the expense of accepting more system noise and gamma-ray background interference, as described in references 1, 10 and 11. Similar cases exist for 6LiF and pure 6Li films. Using an LLD setting of 300 keV, obverse detector irradiation yields maximum thermal neutron-detection efficiencies of 4.3% for 6LiF-coated devices and 11.6% for pure 6Li-coated devices.
Increasing the efficiency can be achieved by intimately attaching two coated devices such that they are facing each other, as shown in FIG. 2. Between the two semiconductor detectors is placed a coating 20 that releases ionizing radiation reaction products 21 upon the interaction with a neutron 22. The ionizing radiation reaction products 21 can then enter into the semiconductor material 23 of either or both detectors, thereby creating a charge cloud 24 of electrons and “holes,” which can be sensed to indicate the occurrence of a neutron interaction within the neutron sensitive film 20. The charges are swept through the detector through the application of a voltage, much like the case shown in FIG. 1, which is applied through the use of conductive contacts 25 upon the surfaces of the semiconductor detector, where the surfaces are generally parallel to each other.
The design does not rely on the full depletion of the detectors and can be operated with modest operating voltages. The most straightforward method for producing such a device is to simply fasten two front-coated devices together. If the neutron reaction film thickness is thin, coincident charged particle emissions from a single neutron absorption event can be measured simultaneously by both detectors if operated individually, thus giving rise to the erroneous recording of two neutron interaction events when only one actually occurred. Erroneous “double counts” can be eliminated by connecting both devices to a single preamplifier, in which a single event always registers as only one count on the preamplifier circuit. Thermal neutron-detection efficiencies of 24% can be reached for pure 6Li-coated sandwich devices.
Morphological Improvements for Improved Efficiency
Neutron-detection efficiency is limited for single coated devices since charged particle reaction products have limited ranges in the neutron reactive thin film. The detector design of FIG. 3 and U.S. Pat. No. 6,545,281 addresses two methods to improve neutron-detection efficiency, both using morphological alterations: (1) to increase the overall surface area of the device, and (2) to increase the statistical probability that the charged particle reaction product will enter the detector sensitive region.
Tiny holes (only one of which is shown in FIG. 3 at 30) may be etched into the top substrate surface, as described in U.S. Pat. No. 6,545,281, using reactive etching techniques, a method that allows for very precise and accurate control of miniature dimensions, as also described in reference 12. Holes etched into the top surface of a material are filled with the neutron reactive material 32, which serves to increase the probability that charged particle reaction products will enter the active region of the detector 34.
From the previous discussion, the maximum probability that a single charge particle product can enter the detector active region is 50%, which corresponds to neutron absorption events that occur at the 10B film/detector interface. By etching a trench into the substrate, the charged particle entrance probability is increased for absorption events that occur in the trench region. Hence, by simply etching trenches into the substrate material before administering the metal contacts 36 and the neutron-sensitive films, the overall detection efficiency of the device can be increased. Yet, charged particle reaction products can still be emitted in trajectories parallel or nearly parallel to the trenches, thereby never coming into contact with the active semiconductor detector 34. The difficulty is resolved by making the trenches circular in shape, or rather, tiny holes that are etched into the device surface. Holes etched into the surface of a material can be filled with the neutron reactive material, which serves to increase the probability that charged particle reaction products will enter the active region of the detector.
Tiny holes can be precisely etched into semiconductors with very high aspect ratios (exceeding 10:1) with various dry etching techniques such as reactive ion etching (RIE), as described in reference 12. The processes use precision photolithography and VLSI thin film techniques, hence the placement of tiny holes is straightforward. Preliminary calculations indicate that the tiny hole diameters should be on the order of the total added charged particle range. The hole sizes are theoretically optimized by making their working diameters exactly the same total effective length L for both charged particle reaction products. Using 10B as an example, the hole diameters should be approximatelyD=LLi+Lα,such that no matter what lateral direction that the charged particles are emitted, one or the other particle will enter the detector. For instance, the value of LLi+Lα=3.458 microns in pure 10B, hence the optimized hole diameters should be approximately 3.5 microns for 10B coated devices. It follows that the hole diameters should be approximately 30 microns for 6LiF films and 100 microns for 6Li films. Etching tiny holes with depths up to 12 microns can be done simply for hole diameters of only 3.5 microns, an aspect ratio of less than 3.5:1. The vertical direction can be optimized from equations 1a and 1b. Still, charged particles can escape detection under some circumstances, but, as depicted in FIG. 3, the probability of one or the other charged particle reaction product entering the detector is tremendously increased. The entire upper surface may be processed such that an optimized matrix of holes covers the entire device upper surface, as shown in U.S. Pat. No. 6,545,281.
The actual contact on the semiconductor devices can be produced by various means, including implantation as described in reference 13, epitaxial growth as described in references 14 and 15, and evaporation or sputtering as described in references 13–15. All of these methods have been explored and developed. The contacts can be made with the low voltage self-biased design, as described in reference 16, or the highly radiation hard Schottky barrier design, as described in references 1 and 17.